Right folks, as I eluded to in my newb post, I've been having some compression issues with the forward cylinder. I've stripped the head off, cleaned up the mating surfaces and reassembled, unfortunately gas was still being pushed out from the joint between the head and barrel I've stripped it down again and I'm going to get them refinished by a machine shop, there's a notable deformation on the raised surface of the barrel which I think could be the issue. Does anyone know what the machining limits are on these heads and barrels? A secondary issue that I've found is that one of the head studs was loose, could this have been part of the cause? If so what is the correct procedure for reinstallation of the stud, does it need to be torqued or just hand tight? All thoughts much appreciated!
Hi, does the leak coincide with the loose stud quarter? Unless the face(s) are very badly deformed, you can usually lap the faces in with valve grinding paste by revolving head on removed barrel (changing relative position between the two regularly). There is no special procedure for installing cylinder stud, some Loctite it's up to you. As long as stud is in good condition and installed as deeply as the other three ( or as deep as possible but without 'going mad' on tightening) then it will be fine.
You will have to experiment with the lapping procedure Jim, I am sure you will get the knack. You might have to use coarse paste to start with but watch out for making deep radial grooves due to the large particles in the paste plus check that most of the paste (and material removal) is on the clamping faces rather than the diameter of the sealing ridge.
Well, I gave it a bash and the surfaces are looking waaaayyyy better than they were before! Just waiting on new head nuts before trying to see if my hard work has paid off...
Good news Jim, you could try using a felt tip pen on both faces and giving it a good rotation without paste just to confirm you have achieved full surface contact. It also just occurred to me that it may be possible to insert studs deeper than necessary if you followed my earlier post to the letter so just measured one and installed height is around 136 to 138 mm just to be on the safe side in case the remaining 3 don't give a good guide line. There are often 2 to 3 threads showing above cylinder head nut when installed which is ideal but you wouldn't want to assemble it all up and find the stud was 'underflush'.
Thanks Chris, studs are all looking OK, cleaned out the thread with a tap and stud went in to the same level at the rest, head contact looked good but I'll do the ink check too. Fingers crossed!!! Thanks for the assistance, very much appreciated
felt tip ink is obviously not as good as engineers blue but it's something everyone has available - sometimes it's possible that the felt tip ink will adhere too well - i.e. will not be removed consistently when you try and rub it off even if faces are acceptable but with careful inspection/repeating process it will still help to give you a guideline as to how good the faces are re: low spots/high spots.
Engineers' Blue..........I have some of that............amazing stuff.............you can pick up the clean tin and put it down again......... .......and in doing that, your fingers look as though you have just had your dabs printed by Plod. Mind you, buy a new tin of bitumen paint from your local DIY shop and by the time you get it home, it's all over the handle.
You don't really need full surface contact - just as long as the top of the spigot on the barrel mates with the bottom of the recess in the head - that's where the gas seal is made. In fact, before you nip the head down, there should be a minute gap between the two surfaces outside of the spigot/recess, to ensure the spigot makes contact first.
Yeah, I've got good contact on the cylinder sealing face, there's no contact on the outer face as it stands, just the o-rings for the oil passages
When the engine was new that was the case, I don't know how many of these you have stripped down but I am here to tell you that as the mating areas get eroded (particularly the outer periphery/diameter being the question in point) over the years the two clamping faces are the ONLY thing that stops compression leaking away. Jim has just confirmed this on his engine in post above.
I fully respect that, but after reading "you don't really need full contact surface" I had to explain to you the true reality of the situation after stripping and observing cylinder/head condition after a few years. Have another look next time and hopefully you will see what I mean.
Well, I tried. Ink looked good, studs all good, belts all timed up and I think it's still blowing. Unsure as low battery so not turned over at full speed but not looking good. What are my options, don't think I can lap any more as I'll end up with a groove in the head.
If faces were definitely flat then a bit of a mystery. It's still worth starting it up to see how bad plus things could improve as a result of heating through. I did a 750 once that leaked slightly on re-assembly but sealed itself after a week or so.
The saga continues! Definitely a leak there but a lot better than it was and as Chris has eluded to hoping the a few heat cycles and it may sort itself out (wishful thinking is my forte!). However I can't get it started as there is now no spark, not a jot, not even a flicker. Checked resistances for pick-up, primary and secondary coils and plug caps and all are in spec according to the Haynes manual. Tried new plugs and old, battery voltages are good and it's cranking strongly. So answers on a postcard! All I can think of is cleaning connections.
